Empirically determine value category of C++11 expression?

Question

Each expression in C++11 has a value category. One of lvalue, xvalue or prvalue.

Is there a way to write a macro that, given any expression as an argument, will produce a string "lvalue", "xvalue" or "prvalue" as appropriate?

For example:

int main()
{
    int x;

    cout << VALUE_CAT(x) << endl; // prints lvalue
    cout << VALUE_CAT(move(x)) << endl; // prints xvalue
    cout << VALUE_CAT(42) << endl; // prints prvalue
}

How could VALUE_CAT be implemented?

Answer 1

You could also try using the clang API's Classification function to return the category of the expression from a clang AST containing the expression. This is, of course, far more complex than @Luc's solution, since it requires generating the actual AST through clang.

Answer 2

#ifndef _TPF_TYPE_NAME_H
#define _TPF_TYPE_NAME_H

template <typename T>
constexpr bool is_lvalue_helper = std::is_lvalue_reference<T>::value;

template <typename T>
constexpr bool is_xvalue_helper = std::is_rvalue_reference<T>::value;

template <typename T>
constexpr bool is_prvalue_helper = !(is_lvalue_helper<T> || is_xvalue_helper<T>);

template <typename T>
constexpr bool is_rvalue_helper = is_xvalue_helper<T> || is_prvalue_helper<T>;

template <typename T>
constexpr bool is_glvalue_helper = is_xvalue_helper<T> || is_lvalue_helper<T>;

#define is_lvalue(type_instance) is_lvalue_helper<decltype((type_instance))>
#define is_xvalue(type_instance) is_xvalue_helper<decltype((type_instance))>
#define is_prvalue(type_instance)is_prvalue_helper<decltype((type_instance))>
#define is_rvalue(type_instance) is_rvalue_helper<decltype((type_instance))>
#define is_glvalue(type_instance)is_glvalue_helper<decltype((type_instance))>

#endif // end of _TPF_TYPE_NAME_H